Several existing printing systems utilize a printing plate in which the areas to print are raised above the areas that are not to be printed. The difference in height of the two levels is called "relief" in the printing field. The amount of relief required is largely dependent on the material to be printed. Media which has little variation in thickness and which can be well controlled need little relief while less precise materials require more relief. An example of highly precise media is magazine grade paper printed in a web. Less precise media is represented for example by corrugated paperboard printed in association with a sheet feeding apparatus. In any case, the required relief is not large. Corrugated paperboard is commonly printed with 1 mm (0.040") relief.
On rotary printing machines, there are several ways to create a print cylinder with these types of printing and non printing areas. One method uses replaceable cylinders that are machined with machine tools. Another method utilizes printing plates that are formed photographically in a flat form, glued to a carrier sheet and then mounted to a cylinder which is sufficiently smaller than the desired printing diameter to account for the thickness of the plate and the carrier sheet. The assembly of the plate, the carrier sheet and provision for mounting it on the cylinder is called a printing blanket. The latter system is the one commonly used to print in machines for making corrugated boxes.
In either case, the cylinder or blanket must be installed and removed for each print job. They must be cleaned and stored between runs of the print job. Changes in the image to be printed involve reworking the cylinder or blanket.
Variations in the thickness of the materials used to make the blanket, plate, adhesive and carrier sheet, can cause the overall thickness of the blanket assembly to vary considerably thereby causing the printing surface to vary from the desired cylindrical surface.